Cuprite: Onganja, Namibia. (25.82 cts). Photo © Joel E. Arem, PhD, FGA. Used with permission.
One of the rarest of all facetable gems, cut cuprites can show magnificent deep red color. However, these beautiful stones are too fragile for jewelry use.
Poorly cut cuprites will appear very dark and dull. Quality cutting will bring out their striking red colors and near-metallic luster, boosting their value.
Custom triangle-cut cuprite, 4.60 cts, 10 x 9.5 mm, Onganja, Namibia. © The Gem Trader. Used with permission.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
| Faceted | 4 carats plus |
| to /ct |
| Data | Value |
|---|---|
| Name | Cuprite |
| Colors | Brownish red, red, purplish red, nearly black. |
| Crystallography | Isometric. Crystals, cubes, and octahedra, or combinations; also needlelike, in densely packed mats called chalcotrichite. |
| Refractive Index | 2.848 |
| Luster | Adamantine to submetallic; earthy. |
| Hardness | 3.5-4 |
| Fracture | Conchoidal to uneven |
| Specific Gravity | 6.14; Namibia: 6.0-6.07. |
| Birefringence | None. |
| Cleavage | Poor |
| Luminescence | None. |
| Luminescence Present | No |
| Transparency | Transparent to opaque. |
| Formula |
Cu2O |
| Pleochroism |
Sometimes anomalously pleochroic. |
| Optics |
N = 2.848. |
| Etymology |
From the Latin cuprum for “copper,” in allusion to its composition. |
| Occurrence |
Secondary mineral in copper deposits. Usually microscopic crystals. |
Cuprites on matrix, 4.3 x 3.5 x 2.5 cm, Mashamba West Mine, Kolwezi, Katanga, Democratic Republic of the Congo. © Rob Lavinsky, www.iRocks.com. Used with permission.
Crystals of cuprite (copper oxide) are typically too small and opaque for faceting. Cuprite often occurs mixed with other copper-bearing minerals, such as malachite, chrysocolla, and azurite. Lapidaries can carve these mixed stones into cabochons.
“Sonoran Sunrise” cabochons, red orange cuprite with blue-green chrysocolla and black tenorite, 22 and 22.5 cts, Milpillas, Sonora, Mexico. © 49erMinerals. Used with permission.
Cuprite has been mined as a copper ore and can also form as a patina on copper and bronze artifacts.
This gilt bronze figure from Han dynasty China (206 BCE-220 CE) has patinas of both cuprite and malachite. Photo courtesy of liveauctioneers.com and Tremont Auctions.
Burnite is a mixture of azurite and cuprite. Chalcotrichite is a dense, fibrous variety of cuprite. Tile ore is a brick-red, massive variety.
Chalcotrichite on crystalized copper, 3.3 x 2.6 x 1.7 cm, Emke Mine, Onganja, Seeis, Windhoek District, Khomas Region, Namibia. © Rob Lavinsky, www.iRocks.com. Used with permission.
Cuprite often pseudomorphs into malachite, which means its chemistry changes to malachite while retaining cuprite’s external crystal form.
Set of cuprites partially pseudomorphed into malachites, Chessy-les-Mines, Rhone, Rhone-Alpes, France. © Rob Lavinsky, www.iRocks.com. Used with permission.
In reflected light, cuprites may appear almost blueish. In transmitted light (when backlit), cuprites can show their deep red colors.
When backlit, these cuprites show their prized, deep red color. Cuprites and chrysocolla on calcite, 5.8 x 5.4 x 5.3 cm, Mashamba West Mine, Kolwezi District, Katanga Copper Crescent, Democratic Republic of the Congo. © Rob Lavinsky, www.iRocks.com. Used with permission.
Cuprite’s very high refractive index (RI) of 2.848 and specific gravity (SG) of 6-6.14 will usually easily help distinguish cuprites from gemstones with similar color and appearance, especially more popular red gemstones like garnets, rubies, and spinels. However, other rare red gemstones of similar appearance — such as proustites, rutiles, and wulfenites — also have over-the-limit (OTL) RIs that overlap or approximate cuprite’s. SG measurements should still distinguish these gems.
Custom square-cut cuprite, 16.48 cts. 13.5 mm, Onganja, Namibia. © The Gem Trader. Used with permission.
Cuprites have a brownish red streak. Please note that streak testing is destructive and should only be conducted as a last resort for identification and never on a finished gem.
As isometric crystals, cuprites have no birefringence or pleochroism, but some stones may show these properties anomalously. Polishing faceted cuprites with diamond paste may leave surface deformations that cause anomalous birefringence. One study has found that polishing with alkaline silica solutions instead can avoid this result.
Scientists have synthesized cuprites for many purposes, including research into anti-fouling paints, the removal of patinas from bronze archeological finds, and other chemical and mineralogical processes. Crystals have also been synthesized, some have even been faceted. However, there doesn’t appear to be widespread use of this lab-created material for jewelry.
Artisans have used cuprite to color glass beads since ancient times, and the practice continues into modern times, with synthetic material added to devitrified glass to simulate gemstones. Iimori Laboratory has produced “Maple Stones,” brownish green devitrified glass pieces that resemble bloodstones with red “flowers” of synthetic cuprite.
Cuprites generally receive no treatments or enhancements.
As a mineral, cuprite occurs in many locations worldwide. However, to date, only one locality — Onganja, Namibia — has produced blood red crystals transparent and large enough to facet. (Most material from this mine is still opaque and often coated with a layer of malachite). Now closed and flooded, this mine probably won’t produce any more cuprite. Since mineral collectors prize these fine crystals and don’t want to see them cut, the availability of facetable material in the marketplace is very limited.
Other notable sources of crystals include the following locations:
Crust of cuprites, 7.4 x 3.8 x 2.0 cm, Chino Mine (Santa Rita Pit; Santa Rita Mine), Santa Rita, Santa Rita District, Grant Co., New Mexico, USA. © Rob Lavinsky, www.iRocks.com. Used with permission.
Cuprites, 3.1 x 2.8 x 1.9 cm, El Cobre Mine, Santiago de Cuba, Sierra Maestra Mts, Oriente Province, Cuba. © Rob Lavinsky, www.iRocks.com. Used with permission.
Before the amazing Onganja discovery in 1973, the largest known cuprites weighed less than 1 carat. Onganja cuprites may reach sizes of 6″ across or more, and lapidaries have cut flawless stones up to 300 carats. Even larger finished stones may be possible.
The largest cuttable, completely transparent mass of cuprite resides in a private collection and weighs 2 kg.
Custom oval-cut cuprite, 54.45 cts, ~ 23.6 x 17.6 mm, Onganja, Namibia. © The Gem Trader. Used with permission.
With a hardness of 3.5 to 4, cuprites scratch easily and aren’t very suitable for jewelry use. Over time, they’ll also lose their color from exposure to light. Therefore, use protective settings for any cuprite jewelry pieces and reserve them for occasional evening wear. Clean these gems only with water, mild detergent, and a soft brush.
Due to its copper content, cuprite dust is toxic. Accidental ingestion could lead to acute distress, like vomiting, and chronic exposure could lead to liver and kidney damage. Faceters should wear protective masks and, ideally, use a glovebox to prevent inhaling or ingesting cuprite particles during cutting, polishing, and cleaning. Wearing or handling finished pieces should pose no hazards.
Consult our gemstone jewelry cleaning guide for more care recommendations.
Cuprite: Onganja, Namibia (48.07 cts). Photo © Joel E. Arem, PhD, FGA. Used with permission.
IGS Newsletter